Asthma is a complex inflammatory condition that afflicts 10-15 million people in the US alone. Asthma and its related phenotypes (airway hyper responsiveness (AHR), atopy and airway inflammation) have a heritable component. Despite this recognition, the genetic mechanism underlying susceptibility to asthma and asthma-related phenotypes remain unknown. Identification of susceptibility genes for human asthma has been hampered by variability in clinical phenotype, genetic heterogeneity in human populations and experimental difficulty in addressing the molecular mechanism underlying complex pathological processes in humans. Thus our strategy is to take advantage of the genetic and experimental tractability of inbred murine models of asthma to provide high quality positional candidates for human genetic studies. Using classic genetic analysis and gene profiling techniques, we have identified complement factor 5 (C5) as a strong positional candidate for one of the two distinct QTLs identified for AHR in the murine model. We provide evidence that an allelic variant in the C5 locus is closely associated with susceptibility to AHR in this murine model. Mechanistically, our preliminary data suggest that expression of the variant allele results in a defective C5 protein leading to impaired IL-12 production and consequent up regulation of Th2 cytokines and AHR. Further support for this possible importance of this locus to asthma is the fact that our collaborators have reported linkage of asthma to the syntenic region of C5 in humans (chr 9q). Thus we plan to critically examine the hypothesis that polymorphism(s) in the C5 region or related genes contribute to the development of asthma or asthma related phenotypes in mice and humans. Our specific aims are (1) to identify additional positional candidate genes for the allergen induced AHR phenotypes from the larger number of candidate genes identified by linkage analysis and preliminary expression studies, we will determine transcription and translation kinetics for positional candidate genes and search for polymorphisms in differentially regulated genes (2) to investigate the role of C5 in susceptibility to AHR, we will manipulate C5 levels in vivo, explore the relationship between C5 expression and AHR in mice with functionally different C5 alleles and explore interactions between C5 and other linked positional candidate genes (3) to determine the mechanism by which altered C5 expression confers susceptibility to AHR, we will test the hypothesis that C5 expression regulates production of the critical Th-1 inducing cytokine IL-12 leading to skewed T cell responses to allergen exposure and subsequent development of AHR (4) to test for association of any identified sequence variants with AHR, elevated total serum IgE and asthma. We will confirm our findings in two additional groups of asthmatics (1) a Caucasian population on Tangier Island, VA and (2) and Afro-Caribbean population from Barbados. This combined (mouse and human) approach to the study of asthma genetics should provide insight into the genes controlling susceptibility to human asthma and promote the development of novel therapeutics.